Summary: Title: Structural Basis of G-protein selectivity in GPCRs using Multiscale Dynamics Upon binding to agonists G protein-coupled receptors (GPCRs) mediate multiple signaling pathways by coupling to intracellular transducer proteins such as G proteins and/or ?-arrestins. Certain agonists exhibit selectivity in their efficacy to specific G-protein signaling pathways. Such selective ligands provide precise therapeutic benefits with fewer side effects as drugs compared to GPCR-targeted drugs in the market. There are very few G-protein selective GPCR agonists known to date, because designing G-protein selective agonists is a daunting experimental challenge. Additionally, there is a serious lack of understanding of structural information on how GPCRs modulate their functional selectivity for their cognate G-protein in cells. There are several contributing factors to how an agonist- GPCR pair shows selectivity to specific G-protein. These factors include, nature of conformational ensembles of the agonist-GPCR-G-protein complexes, and several cellular factors. Delineating the contribution from the structural ensemble of the agonist-GPCR-G-protein complexes has been sparse due to huge experimental challenges in crystallography and NMR of these complexes. We propose to combine two state-of-the-art dynamics techniques, such as ensemble based multi-resolution molecular dynamics method tightly integrated in an iterative fashion with scalable genetically coded FRET sensor biophysical measurements in live cells, to probe the structural basis of G-protein selectivity. The scalability of these two techniques is a huge advantage to probe the functional selectivity of several agonist-GPCR pairings. We propose to use the combination of these two techniques to (a) identify the structural determinants in the agonist-GPCR complex that contribute significantly to G-protein selectivity in nine different agonist-GPCR pairs. (b) We also propose to delineate the structural determinants that contribute to functional selectivity when the GPCR is bound to a partial agonist as opposed to a full agonist, and when the agonist-GPCR is also bound to an allosteric modulator. The outcome of the proposed work will enable structure based design of selective agonists for class A GPCRs, and also provide an understanding of the biological process of how GPCRs recognize their cognate G-proteins in live cells.